Design of synchronization signals for narrowband operation

ABSTRACT

Aspects of the present disclosure provide techniques for design of synchronization signals for narrowband operation, which can be used for stand-alone/in-band/guard-band deployment. An example method is provided for operations which may be performed by a base station (BS). The example method generally includes generating a primary synchronization signal (PSS) utilizing a first code sequence and a cover code applied to the first code sequence over a first number of symbols within one or more subframes, generating a secondary synchronization signal (SSS) based on a second code sequence over a second number of symbols within one or more subframes, and transmitting the PSS and the SSS in the first and second subframes to a first type of a user equipment (UE) that communicates on one or more narrowband regions of wider system bandwidth.

CROSS-REFERENCE TO RELATED APPLICATIONS & PRIORITY CLAIM

The present Application for Patent is a continuation of U.S.Non-Provisional application Ser. No. 15/247,355, filed Aug. 25, 2016,which claims priority to U.S. Provisional Patent Application No.62/234,554, filed Sep. 29, 2015, each of which is assigned to theassignee of the present application and hereby expressly incorporated byreference herein in its entirety.

BACKGROUND Field of the Disclosure

Certain aspects of the present disclosure generally relate to wirelesscommunications and, more particularly, to design of synchronizationsignals for narrowband operation.

Description of Related Art

Wireless communication systems are widely deployed to provide varioustypes of communication content such as voice, data, and so on. Thesesystems may be multiple-access systems capable of supportingcommunication with multiple users by sharing the available systemresources (e.g., bandwidth and transmit power). Examples of suchmultiple-access systems include code division multiple access (CDMA)systems, time division multiple access (TDMA) systems, frequencydivision multiple access (FDMA) systems, 3rd Generation PartnershipProject (3GPP) Long Term Evolution (LTE)/LTE-Advanced systems andorthogonal frequency division multiple access (OFDMA) systems.

Generally, a wireless multiple-access communication system cansimultaneously support communication for multiple wireless terminals.Each terminal communicates with one or more base stations viatransmissions on the forward and reverse links. The forward link (ordownlink) refers to the communication link from the base stations to theterminals, and the reverse link (or uplink) refers to the communicationlink from the terminals to the base stations. This communication linkmay be established via a single-input single-output, multiple-inputsingle-output or a multiple-input multiple-output (MIMO) system.

A wireless communication network may include a number of base stationsthat can support communication for a number of wireless devices.Wireless devices may include user equipments (UEs). Some examples of UEsmay include cellular phones, smart phones, personal digital assistants(PDAs), wireless modems, handheld devices, tablets, laptop computers,netbooks, smartbooks, ultrabooks, etc. Some UEs may be consideredmachine-type communication (MTC) UEs, which may include remote devices,such as sensors, meters, location tags, etc., that may communicate witha base station, another remote device, or some other entity. Machinetype communications (MTC) may refer to communication involving at leastone remote device on at least one end of the communication and mayinclude forms of data communication which involve one or more entitiesthat do not necessarily need human interaction. MTC UEs may include UEsthat are capable of MTC communications with MTC servers and/or other MTCdevices through Public Land Mobile Networks (PLMN), for example.

SUMMARY

Certain aspects of the present disclosure provide a method for wirelesscommunications by a base station. The method generally includesgenerating a primary synchronization signal (PSS) utilizing a first codesequence and a cover code applied to the first code sequence over afirst number of symbols within one or more subframes, generating asecondary synchronization signal (SSS) based on a second code sequenceover a second number of symbols within one or more subframes, andtransmitting the PSS and the SSS to a first type of a user equipment(UE) that communicates on one or more narrowband regions of wider systembandwidth.

Certain aspects of the present disclosure provide a method for wirelesscommunications by a user equipment. The method generally includesdetecting, within one or more narrowband regions of wider systembandwidth, a primary synchronization signal (PSS) generated utilizing afirst code sequence and a cover code applied to the first code sequenceover a first number of symbols within one or more subframes, performinga maximum-likelihood time offset estimation based on the PSS, whereinthe time offset is uncorrelated with a frequency offset, performing aninitial time synchronization and a frequency synchronization based onthe PSS, detecting, within the one or more narrowband regions, asecondary synchronization signal (SSS) generated based on a second codesequence over a second number of symbols within the one or moresubframes, and performing at least one of fine time synchronization orfine frequency synchronization based on the SSS, to refine the initialtime and frequency synchronization.

Numerous other aspects are provided including methods, apparatus,systems, computer program products, and processing systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram conceptually illustrating an example of awireless communication network, in accordance with certain aspects ofthe present disclosure.

FIG. 2 shows a block diagram conceptually illustrating an example of abase station in communication with a user equipment (UE) in a wirelesscommunications network, in accordance with certain aspects of thepresent disclosure.

FIG. 3 shows an exemplary frame structure for frequency divisionduplexing (FDD) in long term evolution (LTE).

FIG. 4 shows two exemplary subframe formats with the normal cyclicprefix.

FIG. 5 illustrates example operations that may be performed by a basestation (BS), in accordance with certain aspects of the presentdisclosure.

FIG. 6 illustrates example operations that may be performed by a userequipment (UE), in accordance with certain aspects of the presentdisclosure.

FIG. 7 illustrates a block diagram of example operations for narrowbandinternet-of-things (NB-IoT) synchronization, in accordance with certainaspects of the present disclosure.

FIG. 8 illustrates an example of dual layer PSS sequence structure, inaccordance with certain aspects of the present disclosure.

FIG. 9 illustrates an example of a modified Zadoff-Chu Sequences withreduced alphabet size, in accordance with certain aspects of the presentdisclosure.

FIG. 10 illustrates a block diagram of example operations that may beperformed by a base station to generate a PSS waveform, in accordancewith certain aspects of the present disclosure.

FIG. 11 is a plot of time-frequency resources illustratingcollision-free resource mapping of NB-IoT and Legacy LTE, in accordancewith certain aspects of the present disclosure.

FIG. 12 illustrates an example timing offsets of maximum-likelihoodestimation (MLE), in accordance with certain aspects of the presentdisclosure.

FIG. 13 illustrates frequency locations of transmitted and received PSSpilots, in accordance with certain aspects of the present disclosure.

FIG. 13A is a graph illustrating normalized frequency offset estimationbased on normalized correlation, in accordance with certain aspects ofthe present disclosure.

FIG. 14 illustrates a block diagram of example operations for timing andfrequency acquisition, in accordance with certain aspects of the presentdisclosure.

FIG. 15 illustrates example operations that may be performed by a basestation to generate a SSS waveform, in accordance with certain aspectsof the present disclosure.

FIG. 16 is a graph illustrating the constellation of PSS sequencecorresponding to a computer generated sequence of length, in accordancewith certain aspects of the present disclosure.

FIG. 16A is a graph illustrating the constellation of PSS sequencecorresponding to a Modified Zadoff-Chu Sequence of length 12, inaccordance with certain aspects of the present disclosure.

FIG. 17 is a graph of timing estimation and timing metric showing theimpacts of cover code optimization on timing offset estimation, inaccordance with certain aspects of the present disclosure.

FIG. 17A is a graph of timing estimation and timing metric showing thedependence of timing offset estimator on the choice of binary cover codepatterns, in accordance with certain aspects of the present disclosure.

FIG. 18 is a table showing the mapping from SSS sequence to cell ID andsystem information, in accordance with certain aspects of the presentdisclosure.

DETAILED DESCRIPTION

Certain aspects of the present disclosure generally relate to design ofsynchronization signals for narrowband operation. Designs presentedherein may be suitable for various narrowband deployment scenarios, suchas both in-band (within wider operating bandwidth) and stand-alonedeployments. As will be described herein, in some cases, a “two-tiered”PSS signal may be constructed, for example, using dual-layer randomcodes with good correlation and run length properties.

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asuniversal terrestrial radio access (UTRA), cdma2000, etc. UTRA includeswideband CDMA (WCDMA), time division synchronous CDMA (TD-SCDMA), andother variants of CDMA. cdma2000 covers IS-2000, IS-95 and IS-856standards. A TDMA network may implement a radio technology such asglobal system for mobile communications (GSM). An OFDMA network mayimplement a radio technology such as evolved UTRA (E-UTRA), ultra mobilebroadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20,Flash-OFDM®, etc. UTRA and E-UTRA are part of universal mobiletelecommunication system (UMTS). 3GPP Long Term Evolution (LTE) andLTE-Advanced (LTE-A), in both frequency division duplex (FDD) and timedivision duplex (TDD), are new releases of UMTS that use E-UTRA, whichemploys OFDMA on the downlink and SC-FDMA on the uplink. UTRA, E-UTRA,UMTS, LTE, LTE-A and GSM are described in documents from an organizationnamed “3rd Generation Partnership Project” (3GPP). cdma2000 and UMB aredescribed in documents from an organization named “3rd GenerationPartnership Project 2” (3GPP2). The techniques described herein may beused for the wireless networks and radio technologies mentioned above aswell as other wireless networks and radio technologies. For clarity,certain aspects of the techniques are described below forLTE/LTE-Advanced, and LTE/LTE-Advanced terminology is used in much ofthe description below. LTE and LTE-A are referred to generally as LTE.

Example Wireless Communication System

FIG. 1 illustrates an example wireless communication network 100, inwhich aspects of the present disclosure may be practiced. For example,techniques presented herein may be used to help UEs and BSs shown inFIG. 1 communicate on a machine type physical downlink control channel(mPDCCH) using a narrowband (e.g., six-PRB) based search space.

The network 100 may be an LTE network or some other wireless network.Wireless network 100 may include a number of evolved Node Bs (eNBs) 110and other network entities. An eNB is an entity that communicates withuser equipments (UEs) and may also be referred to as a base station, aNode B, an access point, etc. Each eNB may provide communicationcoverage for a particular geographic area. In 3GPP, the term “cell” canrefer to a coverage area of an eNB and/or an eNB subsystem serving thiscoverage area, depending on the context in which the term is used.

An eNB may provide communication coverage for a macro cell, a pico cell,a femto cell, and/or other types of cell. A macro cell may cover arelatively large geographic area (e.g., several kilometers in radius)and may allow unrestricted access by UEs with service subscription. Apico cell may cover a relatively small geographic area and may allowunrestricted access by UEs with service subscription. A femto cell maycover a relatively small geographic area (e.g., a home) and may allowrestricted access by UEs having association with the femto cell (e.g.,UEs in a closed subscriber group (CSG)). An eNB for a macro cell may bereferred to as a macro eNB. An eNB for a pico cell may be referred to asa pico eNB. An eNB for a femto cell may be referred to as a femto eNB ora home eNB (HeNB). In the example shown in FIG. 1, an eNB 110 a may be amacro eNB for a macro cell 102 a, an eNB 110 b may be a pico eNB for apico cell 102 b, and an eNB 110 c may be a femto eNB for a femto cell102 c. An eNB may support one or multiple (e.g., three) cells. The terms“eNB”, “base station” and “cell” may be used interchangeably herein.

Wireless network 100 may also include relay stations. A relay station isan entity that can receive a transmission of data from an upstreamstation (e.g., an eNB or a UE) and send a transmission of the data to adownstream station (e.g., a UE or an eNB). A relay station may also be aUE that can relay transmissions for other UEs. In the example shown inFIG. 1, a relay station 110 d may communicate with macro eNB 110 a and aUE 120 d in order to facilitate communication between eNB 110 a and UE120 d. A relay station may also be referred to as a relay eNB, a relaybase station, a relay, etc.

Wireless network 100 may be a heterogeneous network that includes eNBsof different types, e.g., macro eNBs, pico eNBs, femto eNBs, relay eNBs,etc. These different types of eNBs may have different transmit powerlevels, different coverage areas, and different impact on interferencein wireless network 100. For example, macro eNBs may have a hightransmit power level (e.g., 5 to 40 Watts) whereas pico eNBs, femtoeNBs, and relay eNBs may have lower transmit power levels (e.g., 0.1 to2 Watts).

A network controller 130 may couple to a set of eNBs and may providecoordination and control for these eNBs. Network controller 130 maycommunicate with the eNBs via a backhaul. The eNBs may also communicatewith one another, e.g., directly or indirectly via a wireless orwireline backhaul.

UEs 120 (e.g., 120 a, 120 b, 120 c) may be dispersed throughout wirelessnetwork 100, and each UE may be stationary or mobile. A UE may also bereferred to as an access terminal, a terminal, a mobile station, asubscriber unit, a station, etc. A UE may be a cellular phone, apersonal digital assistant (PDA), a wireless modem, a wirelesscommunication device, a handheld device, a laptop computer, a cordlessphone, a wireless local loop (WLL) station, a tablet, a smart phone, anetbook, a smartbook, an ultrabook, etc. In FIG. 1, a solid line withdouble arrows indicates desired transmissions between a UE and a servingeNB, which is an eNB designated to serve the UE on the downlink and/oruplink. A dashed line with double arrows indicates potentiallyinterfering transmissions between a UE and an eNB.

One or more UEs 120 in the wireless communication network 100 (e.g., anLTE network) may also be a narrowband bandwidth UE. These UEs mayco-exist with legacy and/or advanced UEs (e.g., capable of operating ona wider bandwidth) in the LTE network and may have one or morecapabilities that are limited when compared to the other UEs in thewireless network. For example, in LTE Rel-12, when compared to legacyand/or advanced UEs in the LTE network, the narrowband UEs may operatewith one or more of the following: a reduction in maximum bandwidth(relative to legacy UEs), a single receive radio frequency (RF) chain,reduction of peak rate (e.g., a maximum of 1000 bits for a transportblock size (TBS) may be supported), reduction of transmit power, rank 1transmission, half duplex operation, etc. In some cases, if half duplexoperation is supported, the narrowband UEs may have a relaxed switchingtiming from transmit to receive (or from receive to transmit)operations. For example, in one case, compared to a switching timing of20 microseconds (us) for legacy and/or advanced UEs, the narrowband UEsmay have a relaxed switching timing of 1 millisecond (ms).

In some cases, the narrowband UEs (e.g., in LTE Rel-12) may also be ableto monitor downlink (DL) control channels in the same away as legacyand/or advanced UEs in the LTE network monitor DL control channels.Release 12 narrowband UEs may still monitor downlink (DL) controlchannels in the same way as regular UEs, for example, monitoring forwideband control channels in the first few symbols (e.g., physicaldownlink control channel (PDCCH)) as well as narrowband control channelsoccupying a relatively narrowband, but spanning a length of a subframe(e.g., enhanced PDCCH (ePDCCH)).

According to certain aspects, narrowband UEs may be limited to aparticular narrowband assignment of 1.4 MHz or six resource blocks (RBs)partitioned out of the available system bandwidth) while co-existingwithin a wider system bandwidth (e.g., at 1.4/3/5/10/15/20 MHz).Additionally, narrowband UEs may also be able to support one or morecoverage modes of operation. For example, the narrowband UE may be ableto support coverage enhancements up to 15 dB.

As used herein, devices with limited communication resources, e.g.smaller bandwidth, may be referred to generally as narrowband UEs.Similarly, legacy devices, such as legacy and/or advanced UEs (e.g., inLTE) may be referred to generally as wideband UEs. Generally, widebandUEs are capable of operating on a larger amount of bandwidth thannarrowband UEs.

In some cases, a UE (e.g., a narrowband UE or a wideband UE) may performa cell search and acquisition procedure before communicating in thenetwork. In one case, with reference to the LTE network illustrated inFIG. 1 as an example, the cell search and acquisition procedure may beperformed when the UE is not connected to a LTE cell and wants to accessthe LTE network. In these cases, the UE may have just powered on,restored a connection after temporarily losing connection to the LTEcell, etc.

In other cases, the cell search and acquisition procedure may beperformed when the UE is already connected to a LTE cell. For example,the UE may have detected a new LTE cell and may prepare a handover tothe new cell. As another example, the UE may be operating in one or morelow power states (e.g., may support discontinuous reception (DRX)) and,upon exiting the one or more low power states, may have to perform thecell search and acquisition procedure (even though the UE is still inconnected mode).

FIG. 2 shows a block diagram of a design of base station/eNB 110 and UE120, which may be one of the base stations/eNBs and one of the UEs inFIG. 1. Base station 110 may be equipped with T antennas 234 a through234 t, and UE 120 may be equipped with R antennas 252 a through 252 r,where in general T≥1 and R≥1.

At base station 110, a transmit processor 220 may receive data from adata source 212 for one or more UEs, select one or more modulation andcoding schemes (MCS) for each UE based on CQIs received from the UE,process (e.g., encode and modulate) the data for each UE based on theMCS(s) selected for the UE, and provide data symbols for all UEs.Transmit processor 220 may also process system information (e.g., forSRPI, etc.) and control information (e.g., CQI requests, grants, upperlayer signaling, etc.) and provide overhead symbols and control symbols.Processor 220 may also generate reference symbols for reference signals(e.g., the CRS) and synchronization signals (e.g., the PSS and SSS). Atransmit (TX) multiple-input multiple-output (MIMO) processor 230 mayperform spatial processing (e.g., precoding) on the data symbols, thecontrol symbols, the overhead symbols, and/or the reference symbols, ifapplicable, and may provide T output symbol streams to T modulators(MODs) 232 a through 232 t. Each modulator 232 may process a respectiveoutput symbol stream (e.g., for OFDM, etc.) to obtain an output samplestream. Each modulator 232 may further process (e.g., convert to analog,amplify, filter, and upconvert) the output sample stream to obtain adownlink signal. T downlink signals from modulators 232 a through 232 tmay be transmitted via T antennas 234 a through 234 t, respectively.

At UE 120, antennas 252 a through 252 r may receive the downlink signalsfrom base station 110 and/or other base stations and may providereceived signals to demodulators (DEMODs) 254 a through 254 r,respectively. Each demodulator 254 may condition (e.g., filter, amplify,downconvert, and digitize) its received signal to obtain input samples.Each demodulator 254 may further process the input samples (e.g., forOFDM, etc.) to obtain received symbols. A MIMO detector 256 may obtainreceived symbols from all R demodulators 254 a through 254 r, performMIMO detection on the received symbols if applicable, and providedetected symbols. A receive processor 258 may process (e.g., demodulateand decode) the detected symbols, provide decoded data for UE 120 to adata sink 260, and provide decoded control information and systeminformation to a controller/processor 280. A channel processor maydetermine RSRP, RSSI, RSRQ, CQI, etc.

On the uplink, at UE 120, a transmit processor 264 may receive andprocess data from a data source 262 and control information (e.g., forreports comprising RSRP, RSSI, RSRQ, CQI, etc.) fromcontroller/processor 280. Processor 264 may also generate referencesymbols for one or more reference signals. The symbols from transmitprocessor 264 may be precoded by a TX MIMO processor 266 if applicable,further processed by modulators 254 a through 254 r (e.g., for SC-FDM,OFDM, etc.), and transmitted to base station 110. At base station 110,the uplink signals from UE 120 and other UEs may be received by antennas234, processed by demodulators 232, detected by a MIMO detector 236 ifapplicable, and further processed by a receive processor 238 to obtaindecoded data and control information sent by UE 120. Processor 238 mayprovide the decoded data to a data sink 239 and the decoded controlinformation to controller/processor 240. Base station 110 may includecommunication unit 244 and communicate to network controller 130 viacommunication unit 244. Network controller 130 may include communicationunit 294, controller/processor 290, and memory 292.

Controllers/processors 240 and 280 may direct the operation at basestation 110 and UE 120, respectively. For example, processor 280 and/orother processors and modules at UE 120, may perform or direct operations600 shown in FIG. 6. Memories 242 and 282 may store data and programcodes for base station 110 and UE 120, respectively. A scheduler 246 mayschedule UEs for data transmission on the downlink and/or uplink.

FIG. 3 shows an exemplary frame structure 300 for FDD in LTE. Thetransmission timeline for each of the downlink and uplink may bepartitioned into units of radio frames. Each radio frame may have apredetermined duration (e.g., 10 milliseconds (ms)) and may bepartitioned into 10 subframes with indices of 0 through 9. Each subframemay include two slots. Each radio frame may thus include 20 slots withindices of 0 through 19. Each slot may include L symbol periods, e.g.,seven symbol periods for a normal cyclic prefix (as shown in FIG. 3) orsix symbol periods for an extended cyclic prefix. The 2 L symbol periodsin each subframe may be assigned indices of 0 through 2 L−1.

In LTE, an eNB may transmit a primary synchronization signal (PSS) and asecondary synchronization signal (SSS) on the downlink in the center ofthe system bandwidth for each cell supported by the eNB. The PSS and SSSmay be transmitted in symbol periods 6 and 5, respectively, in subframes0 and 5 of each radio frame with the normal cyclic prefix, as shown inFIG. 3. The PSS and SSS may be used by UEs for cell search andacquisition and may contain, among other information, the cell ID alongwith an indication of the duplexing mode (e.g., with PSS and SSS used indetermining the cell ID). The indication of the duplexing mode mayindicate whether the cell utilizes a time division duplexing (TDD) orfrequency division duplexing (FDD) frame structure. The eNB may transmita cell-specific reference signal (CRS) across the system bandwidth foreach cell supported by the eNB. The CRS may be transmitted in certainsymbol periods of each subframe and may be used by the UEs to performchannel estimation, channel quality measurement, and/or other functions.The eNB may also transmit a physical broadcast channel (PBCH) in symbolperiods 0 to 3 in slot 1 of certain radio frames. The PBCH may carrysome system information. The eNB may transmit other system informationsuch as system information blocks (SIBs) on a physical downlink sharedchannel (PDSCH) in certain subframes. The eNB may transmit controlinformation/data on a physical downlink control channel (PDCCH) in thefirst B symbol periods of a subframe, where B may be configurable foreach subframe. The eNB may transmit traffic data and/or other data onthe PDSCH in the remaining symbol periods of each subframe.

Channel quality measurements may be performed by a UE according to adefined schedule, such one based on the DRX cycle of the UE. Forexample, a UE may attempt to perform measurements for a serving cell atevery DRX cycle. The UE may also attempt to perform measurements fornon-serving neighboring cells. Measurements for non-serving neighborcells may be made based on a different schedule than for serving cellsand the UE may need to tune away from the serving cell to measurenon-serving cells when the UE is in connected mode.

To facilitate channel quality measurements, n eNB may transmit a cellspecific reference signal (CRS) on specific subframes. For example, aneNB may transmit CRS over subframes 0 and 5 for a given frame. Anarrowband UE may receive this signal and measure the average power ofthe received signal, or RSRP. The narrowband UE may also calculate aReceive Signal Strength Indicator (RSSI) based on the total receivedsignal power from all sources. A RSRQ may be also be calculated based onthe RSRP and RSSI.

To facilitate measurements, an eNB may provide a measurementconfiguration to UEs in its coverage area. The measurement configurationmay define event triggers for measurement reporting and each eventtrigger may have associated parameters. When the UE detects a configuredmeasurement event, it may respond by sending a measurement report to theeNB with information about the associated measurement objects. Aconfigured measurement event may be, for example, a measured referencesignal received power (RSRP) or a measured reference signal receivedquality (RSRQ) satisfying a threshold. A time-to-trigger (TTT) parametercan be used to define how long a measurement event must persist beforethe UE sends its measurement report. In this way, the UE can signalchanges in its radio conditions to the network.

FIG. 4 shows two exemplary subframe formats 410 and 420 with the normalcyclic prefix. The available time frequency resources may be partitionedinto resource blocks. Each resource block may cover 12 subcarriers inone slot and may include a number of resource elements. Each resourceelement may cover one subcarrier in one symbol period and may be used tosend one modulation symbol, which may be a real or complex value.

Subframe format 410 may be used for two antennas. A CRS may betransmitted from antennas 0 and 1 in symbol periods 0, 4, 7 and 11. Areference signal is a signal that is known a priori by a transmitter anda receiver and may also be referred to as pilot. A CRS is a referencesignal that is specific for a cell, e.g., generated based on a cellidentity (ID or identification). In FIG. 4, for a given resource elementwith label Ra, a modulation symbol may be transmitted on that resourceelement from antenna a, and no modulation symbols may be transmitted onthat resource element from other antennas. Subframe format 420 may beused with four antennas. A CRS may be transmitted from antennas 0 and 1in symbol periods 0, 4, 7 and 11 and from antennas 2 and 3 in symbolperiods 1 and 8. For both subframe formats 410 and 420, a CRS may betransmitted on evenly spaced subcarriers, which may be determined basedon cell ID. CRSs may be transmitted on the same or differentsubcarriers, depending on their cell IDs. For both subframe formats 410and 420, resource elements not used for the CRS may be used to transmitdata (e.g., traffic data, control data, and/or other data).

The PSS, SSS, CRS and PBCH in LTE are described in 3GPP TS 36.211,entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); PhysicalChannels and Modulation,” which is publicly available.

An interlace structure may be used for each of the downlink and uplinkfor FDD in LTE. For example, Q interlaces with indices of 0 through Q−1may be defined, where Q may be equal to 4, 6, 8, 10, or some othervalue. Each interlace may include subframes that are spaced apart by Qframes. In particular, interlace q may include subframes q, q+Q, q+2 Q,etc., where q∈{0, . . . , Q−1}.

The wireless network may support hybrid automatic retransmission request(HARQ) for data transmission on the downlink and uplink. For HARQ, atransmitter (e.g., an eNB) may send one or more transmissions of apacket until the packet is decoded correctly by a receiver (e.g., a UE)or some other termination condition is encountered. For synchronousHARQ, all transmissions of the packet may be sent in subframes of asingle interlace. For asynchronous HARQ, each transmission of the packetmay be sent in any subframe.

A UE may be located within the coverage of multiple eNBs. One of theseeNBs may be selected to serve the UE. The serving eNB may be selectedbased on various criteria such as received signal strength, receivedsignal quality, pathloss, etc. Received signal quality may be quantifiedby a signal-to-noise-and-interference ratio (SINR), or a referencesignal received quality (RSRQ), or some other metric. The UE may operatein a dominant interference scenario in which the UE may observe highinterference from one or more interfering eNBs.

The focus of traditional LTE design is on the improvement of spectralefficiency, ubiquitous coverage, and enhanced quality of service (QoS)support. Current LTE system downlink (DL) and uplink (UL) link budgetsare designed for coverage of high end devices, such as state-of-the-artsmartphones and tablets, which may support a relatively large DL and ULlink budget.

Thus, as described above, one or more UEs in the wireless communicationnetwork (e.g., wireless communication network 100) may be devices thathave limited communication resources, such as narrowband UEs, ascompared to other (wideband) devices in the wireless communicationnetwork. For narrowband UEs, various requirements may be relaxed as onlya limited amount of information may need to be exchanged. For example,maximum bandwidth may be reduced (relative to wideband UEs), a singlereceive radio frequency (RF) chain may be used, peak rate may be reduced(e.g., a maximum of 100 bits for a transport block size), transmit powermay be reduced, Rank 1 transmission may be used, and half duplexoperation may be performed.

In some cases, if half-duplex operation is performed, narrowband UEs mayhave a relaxed switching time to transition from transmitting toreceiving (or receiving to transmitting). For example, the switchingtime may be relaxed from 20 μs for regular UEs to 1 ms for narrowbandUEs. Release 12 narrowband UEs may still monitor downlink (DL) controlchannels in the same way as regular UEs, for example, monitoring forwideband control channels in the first few symbols (e.g., PDCCH) as wellas narrowband control channels occupying a relatively narrowband, butspanning a length of a subframe (e.g., ePDCCH).

In some systems, for example, in LTE Rel-13, the narrowband may belimited to a particular narrowband assignment (e.g., of no more than sixresource blocks (RBs)) within the available system bandwidth. However,the narrowband may be able to re-tune (e.g., operate and/or camp) todifferent narrowband regions within the available system bandwidth ofthe LTE system, for example, in order to co-exist within the LTE system

As another example of coexistence within the LTE system, narrowband UEsmay be able to receive (with repetition) legacy physical broadcastchannel (PBCH) (e.g., the LTE physical channel that, in general, carriesparameters that may be used for initial access to the cell) and supportone or more legacy physical random access channel (PRACH) formats. Forexample, the narrowband UEs may be able to receive the legacy PBCH withone or more additional repetitions of the PBCH across multiplesubframes. As another example, the narrowband UEs may be able totransmit one or more repetitions of PRACH (e.g., with one or more PRACHformats supported) to an eNB in the LTE system. The PRACH may be used toidentify the narrowband UE. Also, the number of repeated PRACH attemptsmay be configured by the eNB.

The narrowband UE may also be a link budget limited device and mayoperate in different modes of operation (e.g. entailing differentamounts of repeated messages transmitted to the narrowband UE) based onits link budget limitation. For example, in some cases, the narrowbandUE may operate in a normal coverage mode in which there is little to norepetition (i.e., the amount of repetition needed for the UE tosuccessfully receive a message may be low or repetition may not even beneeded). Alternatively, in some cases, the narrowband UE may operate ina coverage enhancement (CE) mode in which there may be high amounts ofrepetition. For example, for a 328 bit payload, a narrowband UE in CEmode may need 150 or more repetitions of the payload in order tosuccessfully receive the payload.

In some cases, for example, for LTE Rel-13, the narrowband UE may havelimited capabilities with respect to its reception of broadcast andunicast transmissions. For example, the maximum transport block (TB)size for a broadcast transmission received by the narrowband UE may belimited to 1000 bits. Additionally, in some cases, the narrowband UE maynot be able to receive more than one unicast TB in a subframe. In somecases (e.g., for both the CE mode and normal mode described above), thenarrowband UE may not be able to receive more than one broadcast TB in asubframe. Further, in some cases, the narrowband UE may not be able toreceive both a unicast TB and a broadcast TB in a subframe.

Narrowband UEs that co-exist in the LTE system may also support newmessages for certain procedures, such as paging, random accessprocedure, etc. (e.g., as opposed to conventional messages used in LTEfor these procedures). In other words, these new messages for paging,random access procedure, etc. may be separate from the messages used forsimilar procedures associated with non-narrowband UEs. For example, ascompared to conventional paging messages used in LTE, narrowband UEs mayable to monitor and/or receive paging messages that non-narrowband UEsmay not able to monitor and/or receive. Similarly, as compared toconventional random access response (RAR) messages used in aconventional random access procedure, narrowband UEs may be able toreceive RAR messages that also may not be able to be received bynon-narrowband UEs. The new paging and RAR messages associated withnarrowband UEs may also be repeated one or more times (e.g., “bundled”).In addition, different numbers of repetitions (e.g., different bundlingsizes) for the new messages may be supported.

According to certain aspects, multiple narrowband regions, with eachnarrowband region spanning a bandwidth that is no greater than a totalof 6 RBs, may be supported by narrowband UE and/or narrowband operation.In some cases, each narrowband UE in narrowband operation may operatewithin one narrowband region (e.g., at 1.4 MHz or 6 RBs) at a time.However, narrowband UEs in narrowband operation, at any given time, mayre-tune to other narrowband regions in the wider system bandwidth. Insome examples, multiple narrowband UEs may be served by the samenarrowband region. In other examples, multiple narrowband UEs may beserved by different narrowband regions (e.g., with each narrowbandregion spanning 6 RBs). In yet other examples, different combinations ofnarrowband UEs may be served by one or more same narrowband regionsand/or one or more different narrowband regions.

Some systems, for example, in LTE Rel-13, introduce coverageenhancements and support for narrowband UEs, as well as other UEs. Asused herein, the term coverage enhancement generally refers to any typeof mechanism that extends the coverage range of a device (such as anarrowband device) within a network. One approach for coverageenhancement (CE) is bundling which refers to transmitting the same datamultiple times (e.g., across multiple subframes or, as will be describedin greater detail below, across multiple symbols within a samesubframe).

In certain systems, narrowband UEs may support narrowband operationwhile operating in a wider system bandwidth. For example, a narrowbandUE may transmit and receive in a narrowband region of a systembandwidth. As noted above, the narrowband region may span 6 resourceblocks (RBs).

Certain systems may provide narrowband UEs with coverage enhancements ofup to 15 dB, which maps to 155.7 dB maximum coupling loss between the UEand an eNB. Accordingly, narrowband UEs and eNB may perform measurementsat low SNRs (e.g., −15 dB to −20 dB). In some systems, coverageenhancements may include channel bundling, wherein messages associatedwith narrowband UEs may be repeated (e.g., bundled) one or more times.

Certain devices may be able to communicate both with legacy typecommunications and non-legacy type communications. For example, somedevices may be able to communicating in both narrowband regions (ofoverall system bandwidth) as well as wider band regions. While theexamples above refer to low cost or MTC devices that communicate vianarrowband regions, other (non-low-cost/non-MTC) types of devices mayalso communicate vie narrowband regions, for example, taking advantageof frequency selectivity and directional transmissions.

Example Design of Synchronization Signals for Narrowband Operation

Certain aspects of the present discourse provide design ofsynchronization signals, which may be detected by devices thatcommunicate using relative narrowband regions of system bandwidth, suchas narrowband internet of things (NB-IoT) devices. As noted above,designs presented herein may utilize a two-tiered approach to generate aPSS signal with good correlation and run length properties that may besuitable for both in-band and stand-alone deployments.

Such synchronization signals may include PSS used for frequency andtiming synchronization and SSS to convey system information. Accordingcertain aspects of the present disclosure, synchronization signals ofnarrowband internet of things (NB-IoT) operations occupy narrow channelbandwidths and can coexist with legacy GSM/WCDMA/LTE system(s) deployedin the same frequency band. In one aspect of the present disclosure, theNB-IoT synchronization signals utilize only one Physical Resource Block(PRB).

According to certain aspects of the present disclosure, narrowbanddevices may supports very low throughput machine to machine (M2M)communications in low-end machine type communications (MTC) market, andit can provide power efficiency. For example, the battery life for somedevices may have a target of up to ten years (e.g., for a batterycapacity of five watt-hours). It is conceivable that a large number ofNB-IoT devices may be deployed and may provide reliable indoor coverageof up to 20 dB coverage extension relative to legacy GPRS. Such devicesmay also be able to operate in challenging deployments, such as groundfloor and basement deployments. Such devices may have reduced complexity(and cost) as they may not need to support circuit switched services andmay not need to support Inter-Radio Access Technology (IRAT) mobility.

According to certain aspects of the present disclosure, in-banddeployments of NB-IoT devices may be in compliance with legacy LTEnumerology (as in compatible tone spacing and bandwidth). As will bedescribed in greater detail below, resource allocation can becollision-free between NB-IoT and legacy LTE DL physical signals, suchas cell-specific reference signals (CRS), synchronization signals, andcontrol channel.

According to certain aspects, the PSS/SSS scheme presented herein mayallow UEs to perform both fractional and integer carrier frequencyoffsets even in deployments with frequency offset up to 20 ppm (e.g.,approximately 18 KHz to a 900 MHz Global System for Mobile (GSM)) forinitial DL synchronization. Because 18 KHz is greater than 15 KHz(NB-IoT subcarrier spacing), the NB-IoT devices are required to handlefrequency offset greater than one subcarrier-spacing.

In some cases (e.g., with an ever increasing number of deployed IoTdevices and the dominance of UL traffic, the synchronization signals arerequired to carry more system information, such as more cell IDs,relative to conventional synchronization signals (e.g., PSS/SSS oflegacy LTE devices). In large scale deployments, NB synchronizationsignals may use more bits allocated for signaling of system informationthan legacy LTE, for example, to signal a subframe index indication,in-band/standalone/guard-band deployment mode, and frequency-divisionduplexed (FDD)/time-division duplexed (TDD) multiplexing code.

PSS/SSS designs presented herein may be suitable forstand-alone/in-band/guard-band deployments, and the design can beextended to other LTE-based MTC deployments using more than one physicalresource block (PRB). For example, synchronization signals subcarrierspacing can be 15 KHz and may be fully aligned with OFDM symbolboundaries of legacy LTE. In some cases, NB-IoT synchronization signalscan utilize resource elements unoccupied by legacy LTE DL physicalsignals and control channels, and the utilization of unoccupied resourceelements can resolve resource segmentation and interference problems.Such signal design may allow for the co-existence of narrow-band andwide-band LTE systems within legacy LTE operation.

For some cases of extreme coverage situations, a minimum coupling loss(MCL) of 164 dB may be needed. A design may have high power efficiency,support a large number of devices, and be implemented at a low cost. Insome cases, a 180 kHz channel bandwidth may be used for communicationsby narrowband UEs.

FIG. 5 illustrates example operations 500 that may be performed by abase station (BS) according to the synchronization signaling inaccordance with aspects of the present disclosure.

The operations 500 begin, at 502, by generating a primarysynchronization signal (PSS) utilizing a first code sequence and a covercode applied to the first code sequence over a first number of symbolswithin one or more subframes. At 504, the base station generates asecondary synchronization signal (SSS) based on a second code sequenceover a second number of symbols within one or more subframes. These twooperations (502 and 504) may correspond to the two-tiered approach togenerating a PSS described above.

At 506, the base station transmits the PSS and the SSS in the first andsecond subframes to a first type of a user equipment (UE) thatcommunicates on one or more narrowband regions of wider systembandwidth. PSS and SSS may be transmitted in any suitable manner, forexample, using non-overlapping resources. In some cases, PSS may be sentfirst and SSS later (e.g., using different symbols in the same subframeor in different subframes).

FIG. 6 illustrates example operations 600 that may be performed by auser equipment (UE), to detect PSS/SSS signaling in accordance withaspects presented herein. In other words, the operations may beperformed to detect PSS/SSS signaling transmitted by a base station inaccordance with the operations 500 of FIG. 5 described above.

The operations 600 begin, at 602, by detecting, within one or morenarrowband regions of wider system bandwidth, a primary synchronizationsignal (PSS) generated utilizing a first code sequence and a cover codeapplied to the first code sequence over a first number of symbols withinone or more subframes.

At 604, the UE performs a maximum likelihood time offset estimationbased on the PSS, wherein the time offset is uncorrelated with afrequency offset. At 606, the UE performs an initial timesynchronization and a frequency synchronization based on the PSS.

At 608, the UE detects, within the one or more narrowband regions, asecondary synchronization signal (SSS) generated based on a second codesequence over a second number of symbols within the one or moresubframes. At 610, the UE performs at least one of fine timesynchronization or fine frequency synchronization based on the SSS, torefine the initial time and frequency synchronization.

FIG. 7 is a flow chart illustrates example operations 700 for NB-IoTsynchronization, in accordance with certain aspects of the presentdisclosure. The operations shown in FIG. 7, for example, may representmore specific examples of the operations described above.

At 702, NB-IoT DL synchronization begins with initial timesynchronization by PSS (auto correlation in the time domain). At 704,fractional frequency offset correction may be performed based on the PSS(auto correlation between co-phased and time synchronized symbolsaccording to a pattern of the cover code, or multiple hypothesis basedon frequency binning). At 706, the PSS may be used to perform integerfrequency offset correction (e.g., based on cross correlation infrequency domain between the PSS and a local/delayed replica of thereceived signal).

At 708, fine timing offset tuning may be performed based on the SSS(cross correlation in time domain). As illustrated, the SSS may be usedto decode not only cell ID 710, but also additional information, such assubframe ID 712, and/or other system information 714. In some cases, apattern of a cover code may be selected to shape the ML estimation ofthe timing offset into a desired form.

According to certain aspects of the present disclosure, PSS can bedesigned with L orthogonal frequency-division multiplexing (OFDM)symbols, where L can be a configurable integer, and the L PSS symbolscan be mapped to localized and/or distributed resource elements. In somecases, all L PSS symbols may be generated by only one Inverse FastFourier Transform (IFFT). As noted above, subcarrier spacing may be 15KHz, and bandwidth may be 180 KHz. In some cases, the PSS may begenerated according to a dual-layer sequence design scheme, using a“Base Code” (for Layer I) and a “Code Cover” (for Layer II).

According to certain aspects of the present disclosure, theconstellation of “Base Code” can be Binary Phase Shift Keying (BPSK), orQuadrature Phase Shift Keying (QPSK), or punctured MPSK modulations withsmall alphabet size and relatively large Euclidean distance. The basedcode sequence is required to possess good auto-correlation properties infrequency domain, which enables reliable detection of integer frequencyoffset.

For example, a PSS “Base Code” may use computer generated sequence(CGS), modified Zadoff-Chu sequence, punctured Zadoff-Chu sequence,m-sequence, quadrature Walsh code, PN sequence or Barker code. A PSSsequence may be constructed by employing binary code cover on top of PSSbase code to improve the accuracy of timing acquisition.

In other aspects of the present disclosure, the “Code Cover” may be abinary random code “cover” with asymmetrical run length (for l's and−1's), and run lengths equal to or greater than two may be used topreserve phase continuity and provides expanded time window forover-sized Fast Fourier Transform (FFT), which may improve frequencyoffset resolution. In one aspect, “Code Cover” polarity reverseasymmetry may help resolving timing ambiguity. In some cases, the codecover may include a Barker code, a Walsh code, or the like. Code coveroptimization can be achieved by computer search for given PSS sequencelength. In some cases, truncated versions of PSS sequences used inlegacy systems (such as LTE) may also be used for NB-IoT PSS sequences.

FIG. 8 illustrates an example of dual layer PSS sequence structure 800,in accordance with certain aspects of the present disclosure. Asillustrated, the dual layer corresponds to mapping L×K localized ordistributed resource elements within one LTE physical resource block(PRB), and that for in-band deployment, NB-IoT allocation can avoid the“reserved” REs of legacy LTE.

FIG. 9 illustrates an example of a modified Zadoff-Chu Sequences 900with reduced alphabet size. The example shows that modified ZC sequencesmay be utilized to reduce phase and frequency errors and maintainingautocorrelation properties of Zadoff-Chu Sequence. The illustratedexample shows that for K=12, base code of PSS sequence can be defined asC(k), where k=1, 2, . . . , K.

FIG. 10 illustrates example logical blocks 1000 for generating a PSSwaveform, according to aspects of the present disclosure. The blocks1000 may be implemented in any suitable circuitry.

As illustrated, the blocks 1000 may include a block 1104 to pad a PSSBase Code with zero(s), which may be required to make length of the IFFTinput into a value that is equal to the power of two. An inverse fastFourier transform (IFFT) block 1006 may take the padded PSS base codeand its output may undergo a parallel to serial conversion at block1008. At block 1010, a cyclic prefix (CP) may be inserted and, at block1012 a code cover may be applied (e.g., using a Binary Pseudo Noise (PN)Code Cover). At block 1014, the signal may be converted to analog signalthrough a digital to analog converter (DAC) and filtered, thentransmitted.

In an example, in order to illustrate a single boundary NB-IoT fullyaligned with legacy LTE, a 128 IFFT size may be chosen, samplingfrequency (Fs) may be set at 1.92 MHz, and subcarrier spacing may be setat 15 KHz (although these parameters may vary). Symbol boundaryalignment between the NB-IoT and the Legacy LTE may help preventinterference that may otherwise occur without the alignment. In certainaspects of the present disclosure, the PSS Base Code and Cover Code canbe generated by the techniques mentioned in current application, and PSSsymbols can occupy resource elements orthogonal to legacy LETallocation.

FIG. 11 illustrates an example representation (resource map 1100) ofcollision-free resource mapping of NB-IoT and Legacy LTE, wherein themapping may be suitable for both in-band and standalone deployment ofNB-IoT. The example shows subframes n and n+1 in the time domain.

The example further shows subframe n may have NB-IoT PSS utilizingsymbol elements that are unoccupied by Legacy LTE, and subframe n+1 mayhave NB-IoT SSS utilizing symbol elements that are unoccupied by LegacyLTE. The example also shows the NB-IoT synchronization signals (PSS/SSSof both in-band and standalone deployments) may be fully aligned withthe Legacy LTE OFDM symbol boundary, and because the symbols may befully aligned, no collision (conflict) and no interference occur betweenthe NB-IoT and Legacy LTE. For more generalized case, the PSS sequenceand SSS sequence may be time multiplexed in the same subframe withoutoverlapping. Repetition of PSS sequence and SSS sequences acrossmultiple subframes is also supported for coverage extension.

FIG. 12 illustrates an example table 1200 illustrating decoupledmaximum-likelihood estimation (MLE) of timing offset as part of a timedomain processing. As illustrated, PSS samples may be received withnoise w, timing offset τ, and frequency offset v.

${r\left( {{n;\tau},v} \right)} = {{\sum\limits_{k}{b_{l,k}\mspace{14mu} {\exp \left( {\frac{j\; {\pi \left( {k + v} \right)}}{64}\left( {n - \tau} \right)} \right)}}} + {w(n)}}$(l − 1)N + l N_(g) < n ≤ l(N + N_(g)), l ∈ {1, 2, . . .  , L}, 1 ≤ k ≤ K ≤ 12

Where N and N_(g) denote the sample size of an OFDM symbol and its CP,respectively. The illustration further shows the boundary of PSS symbolsmay be a function of timing offset τ; and for a given hypothesis ontiming offset τ, γ_(i), the size 1 by N sub-vector of PSS samplescorresponding to symbol i, may also be a function of timing offset τ.Also as illustrated, a metric for decoupled timing offset estimation canbe formed from normalized and coherently combined sub-vectors of PSSsamples; and the metric is independent from frequency offset, whichserves as a cost function for the timing offset (e.g., to reducesensitivity to SNR) and can be expressed by:

${\lambda (\tau)}\overset{\Delta}{=}{\sum\limits_{D = 1}^{({L - 1})}{{\sum\limits_{l = 1}^{L - D}{{s(l)}{s\left( {l + D} \right)}\mathrm{\Upsilon}_{l}\mathrm{\Upsilon}_{l + D}^{H}}}}}$

Where D stands for the symbol distance between two sub-vectors whosephase difference is proportional to DNv, and the product of s(l)s(l+D)is employed to enforce coherent combining for co-phased terms whosephase is proportional to DNv. In some cases, a range of the combiningspans across multiple subframes. In some cases, a lag size of thedelayed samples may be a configurable parameter based on PSS time domainresource allocation. The illustration shows the Maximum-LikelihoodEstimate (MLE) of timing offset is decoupled from the frequency offsetv. Therefore, the timing offset estimation may be obtained by{circumflex over (τ)}=argmax_(τ)λ(τ). In one aspect of the presentdisclosure, MLE of fractional frequency may be derived based on, atleast partly, the decoupled MLE of timing offset.

Alternatively, MLE of fractional frequency offset may be based on, atleast partly, the MLE of timing offset from FIG. 12. In one examplewhere the normalized frequency offset may be 1.2. As illustrated,normalized carrier frequency offset (CFO) can be split into a fractionalpart and an integer part:

v=v _(F) +v _(I), where v _(I)∈

and −0.5≤v _(F)≤0.5

As illustrated in the above equation, v represents the normalizedcarrier frequency offset (CFO), v_(F) represents the fractional part ofthe normalized CFO, and v_(I) represents the integer part of thenormalized CFO. Thus, estimation of frequency offset may be separatedinto two parts: fractional frequency offset estimation (the 0.2 of the1.2 normalized frequency offset, or v_(F)=0.2), and integer frequencyoffset estimation (the 1 of the 1.2 normalized frequency offset, orv_(I)=1).

As illustrated, samples of the second symbol of the PSS symbol pairs{γ₂, γ₃} and {γ₄, γ₅)}, differ from their predecessors by a constantphase shift that is proportional to the fractional part of CFO.Therefore,

$\mspace{20mu} {{{{assume}\mspace{14mu} \mathrm{\Upsilon}_{l}}\overset{\Delta}{=}\left\lbrack {{r\left( {l,1} \right)}\mspace{14mu} {r\left( {l,2} \right)}\mspace{14mu} {{r\left( {l,3} \right)}\mspace{14mu}.\;.\;.\mspace{14mu} {r\left( {l,N} \right)}}} \right\rbrack},\mspace{14mu} {then}}$${\mathrm{\Upsilon}_{l + 1} = {{e^{j\; 2\pi \; v} \times \mathrm{\Upsilon}_{l}} = {{e^{{j\; 2\; \pi \; v}\;}{{F\left\lbrack {r\left( {l,1} \right)\mspace{14mu} {r\left( {l,2} \right)}\mspace{14mu} {{r\left( {l,3} \right)}\mspace{14mu}.\;.\;.\mspace{14mu} {r\left( {l,N} \right)}}} \right\rbrack}}(l)} = {{\frac{1}{2\; \pi}{\arg \left( {\mathrm{\Upsilon}_{l + 1}\mathrm{\Upsilon}_{l}^{H}} \right)}} = {\frac{1}{2\pi \; N}{\sum\limits_{n = 1}^{N}{\arg \left\{ {{s(l)}{s\left( {l + 1} \right)}{r\left( {{l + 1},n} \right)}{r^{*}\left( {l + n} \right)}} \right\}}}}}}}},\mspace{14mu} \mspace{20mu} {l = 2},4$

In one embodiment, samples of symbol γ₃ differ from samples of γ₂ by aconstant phase shift proportional to the fractional part of CFO(j2λv_(F) represents a constant phase rotation proportional only tov_(F)). In some cases, for different PSS symbols, {circumflex over(v)}_(F)(l) can be averaged to obtain a smoother estimate.

In one aspect of the present disclosure, integer part of the normalizedCFO, v_(I), may be estimated by cross correlation of received andtransmitted PSS pilots in the frequency domain because v_(I) leads toshifts in the PSS pilots in the frequency domain from the pilots'original locations.

As illustrated in plots 1302 and 1304 of FIGS. 13 and 13A, the originallocations of the PSS pilots transmitted may be shifted by v_(I) in thefrequency domain, and the shifted locations may be reflected in the PSSpilots received. In this illustration, the peak location of thenormalized correlation where {circumflex over (v)}_(I)=1 correlates tothe normalized integer frequency offset estimate.

In certain aspects of the present disclosure, boundaries of discreteFourier transform (DFT) window can be established with the MLE timingoffset, and leakage from side lobes of fractional CFO can be minimizedafter correction of the fractional offset of the CFO, and thereliability of integer CFO MLE can be obtained through averagingmultiple observations of the cross-correlation. As illustrated in plot1300A of FIG. 13A, an implementation of timing and frequency acquisitionfor an in-band NB-IoT PSS through time domain processing and frequencydomain processing.

In one aspect of the present disclosure, NB-IoT SSS can be designed withM OFDM symbols, where M can be a configurable integer (e.g., greaterthan or equal to six), and the M SSS symbols can be mapped to localizedand/or distributed resource elements. In another aspect of the presentdisclosure, subcarrier spacing for BN-IoT SSS can be 15 KHz, andbandwidth may be 180 KHz. In some cases, the SSS sequence design can begenerated by a Length-K Zadoff-Chu sequence with root u(m) for m-th SSSsymbol where root u(m)∈{1, 2, . . . , K} and gcd(u(m), K)=1, andcomputer generated sequence (CGS) or modified Zadoff-Chu sequences maybe used for SSS sequence. Yet, in another aspect of the presentdisclosure, truncated LTE SSS sequences can be used for NB-IoT SSSsequences. In certain aspects of the present disclosure, cell ID mappingmay be accomplished by one-to-one mapping from decimal to M-elementarray. As illustrated, the number of roots can be equivalent to thenumber of potential candidates for NB-IoT SSS symbol location, andNB-IoT SSS can carry 13-bit, or more, of information for cell ID,subframe ID, and other system information.

FIG. 14 illustrates an example block diagram 1400 for PSS/SSS processingfor timing and frequency synchronization, in accordance with certainaspects of the present disclosure. As illustrated, after taking timedomain samples of PSS, correlation operations may be performed (e.g.,via normalizing and performing a peak search) for timing offsetdetection, symbol boundary alignment and fractional CFO correction(e.g., by phase differentiation). The results may be used to performinteger CFO correction (e.g., by frequency domain correlation) and thecorresponding output used for SSS processing.

FIG. 15 illustrates an example block diagram 1500 for generating an SSSwaveform, according to aspects of the present disclosure. Asillustrated, NB-IoT SSS waveform generation process can be differentthan NB-IoT PSS waveform generation process (e.g., in that there may beno Code Cover applied).

As illustrated, an SSS sequence may be padded with zeroes, followed byan IFFT, parallel to serial conversion followed by cyclic prefix (CP)insertion. In the example, to illustrate a single boundary NB-IoT fullyaligned with legacy LTE, a 128 IFFT size was chosen, the samplingfrequency (Fs) may be set at 1.92 MHz (may vary in other instances), andsubcarrier spacing may be set at 15 KHz (may vary in other instances).The boundary alignment between the NB-IoT and the Legacy LTE preventsinterference that may otherwise occur without the alignment.

The example also shows the SSS sequence can be converted to analogsignal through a digital to analog converter (DAC), and the signal isfiltered and transmitted (TX). In certain aspects of the presentdisclosure, the SSS sequence can be generated by the techniquesmentioned in current application, and SSS symbols can occupy resourceelements (REs) orthogonal to REs per legacy LTE allocation.

FIG. 16 illustrates a graphical example of PSS sequence constellation1600 corresponding to computer generated sequence (CGS) of length 12.The constellation of “Base Code” can be Binary Phase Shift Keying(BPSK), or Quadrature Phase Shift Keying (QPSK), or punctured MPSKmodulations with small alphabet size and relatively large Euclideandistance. The based code sequence may possess suitable auto-correlationproperties in frequency domain, which enables reliable detection ofinteger frequency offset. For example, PSS “Base Code” may use computergenerated sequence (CGS), modified Zadoff-Chu sequence, puncturedZadoff-Chu sequence, m-sequence, quadrature Walsh code, PN sequence orBarker code. CGS may have similarly suitable auto-correlation propertiesin the time domain as a length-K Zadoff-Chu (ZC) sequence and may beconstructed using a reduced-size alphabet with enlarged Euclideandistance. In one aspect of the present disclosure, when K=12, CGS isQPSK.

FIG. 16A illustrates a graphical example of PSS sequence constellation1600A corresponding to modified Zadoff-Chu (ZC) sequence of length 12.ZC sequence may have similarly suitable auto-correlation properties inthe time domain as a CGS. Modified ZC sequence may be a set ofchirp-like polyphase sequences with a minimum number of alphabets. Inone aspect of the present disclosure, when K=12, modified ZC sequencemay uses only five out of six constellation points of 6-PSK, and theconstellation may be generated by puncturing a 6-PSK constellation. Plot1700 of FIG. 17 illustrates an example of impacts of cover codeoptimization on timing offset estimation. The example further shows thedependence of timing offset estimator on the choice of binary cover codepatterns when PSS sequence duration equals 11 symbols.

Plot 1700A of FIG. 17A illustrates an example of dependence of timingoffset estimator on the choice of binary cover code patterns when PSSsequence duration equals 4 symbols.

FIG. 18 illustrates an example of mapping from SSS sequence to cell ID,subframe number, and other system information. The example shows eachSSS sequence may comprises a set of M SSS symbols, and the m-th symbolof the k-th SSS sequence can be constructed by a root (k, m) Zadoff-Chusequence. In one aspect of the present disclosure, the k-th SSS sequencecan be uniquely identified by its roots arrangement.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or combinations thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, software/firmware, or combinations thereof. To clearlyillustrate this interchangeability of hardware and software/firmware,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware orsoftware/firmware depends upon the particular application and designconstraints imposed on the overall system. Skilled artisans mayimplement the described functionality in varying ways for eachparticular application, but such implementation decisions should not beinterpreted as causing a departure from the scope of the presentdisclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in asoftware/firmware module executed by a processor, or in a combinationthereof. A software/firmware module may reside in RAM memory, flashmemory, ROM memory, EPROM memory, EEPROM memory, phase change memory,registers, hard disk, a removable disk, a CD-ROM, or any other form ofstorage medium known in the art. An exemplary storage medium is coupledto the processor such that the processor can read information from, andwrite information to, the storage medium. In the alternative, thestorage medium may be integral to the processor. The processor and thestorage medium may reside in an ASIC. The ASIC may reside in a userterminal. In the alternative, the processor and the storage medium mayreside as discrete components in a user terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software/firmware, or combinations thereof. Ifimplemented in software/firmware, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD/DVD or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software/firmware is transmitted from awebsite, server, or other remote source using a coaxial cable, fiberoptic cable, twisted pair, digital subscriber line (DSL), or wirelesstechnologies such as infrared, radio, and microwave, then the coaxialcable, fiber optic cable, twisted pair, DSL, or wireless technologiessuch as infrared, radio, and microwave are included in the definition ofmedium. Disk and disc, as used herein, includes compact disc (CD), laserdisc, optical disc, digital versatile disc (DVD), floppy disk andBlu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveshould also be included within the scope of computer-readable media.

As used herein, including in the claims, the term “and/or,” when used ina list of two or more items, means that any one of the listed items canbe employed by itself, or any combination of two or more of the listeditems can be employed. For example, if a composition is described ascontaining components A, B, and/or C, the composition can contain Aalone; B alone; C alone; A and B in combination; A and C in combination;B and C in combination; or A, B, and C in combination. Also, as usedherein, including in the claims, “or” as used in a list of items (forexample, a list of items prefaced by a phrase such as “at least one of”or “one or more of”) indicates a disjunctive list such that, forexample, a list of “at least one of A, B, or C” means A or B or C or ABor AC or BC or ABC (i.e., A and B and C).

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for wireless communications by a firsttype of user equipment (UE), comprising: detecting, within one or morenarrowband regions, a primary synchronization signal (PSS) generatedutilizing a first code sequence and a cover code applied to the firstcode sequence over a first number of symbols within one or moresubframes, wherein the cover code comprises a binary sequence;performing a time offset estimation and a frequency offset estimation,based on the PSS, wherein the time offset is uncorrelated with thefrequency offset; and performing an initial time synchronization and afrequency synchronization based on the PSS.
 2. The method of claim 1,wherein performing the time and frequency offset estimations comprisesperforming maximum-likelihood time and frequency offset estimationsbased on the PSS.
 3. The method of claim 1, wherein performing the timeand frequency offset estimations comprises performing amaximum-likelihood frequency offset estimation of a normalized frequencyoffset having a fractional part and an integer part.
 4. The method ofclaim 3, wherein performing the frequency offset estimation comprisesestimating the integer part by a cross correlation of received andtransmitted PSS pilots in frequency domain.
 5. The method of claim 3,wherein performing the frequency offset estimation comprises estimatingthe fractional part based on the time offset estimation.
 6. The methodof claim 3, wherein: performing the time offset estimation comprisesnormalizing and performing a peak search of the time offset; andperforming the frequency offset estimation comprises: aligning symbolboundaries of the PSS; correcting the fractional part by phasedifferentiation; and correcting the integer part by frequency domaincorrelation based on the corrected fractional part and symbolboundaries.
 7. The method of claim 1, wherein performing the frequencysynchronization comprises: performing a fractional frequency offsetcorrection, from the PSS, based on autocorrelation in time domain of thetime synchronized PSS or based on multiple hypothesis testing andfrequency binning; and performing an integer frequency offsetcorrection, from the PSS, based on cross correlation in frequency domainbetween the PSS and a local replica of the received signal.
 8. Themethod of claim 1, further comprising: detecting, within the one or morenarrowband regions, a secondary synchronization signal (SSS) generatedbased on a second code sequence over a second number of symbols withinthe one or more subframes; and performing at least one of fine timesynchronization or fine frequency synchronization based on the SSS, torefine the initial time and frequency synchronization.
 9. The method ofclaim 8, wherein performing the fine time synchronization comprises:performing a timing offset correction based on a maximum likelihood (ML)estimation by coherently combining all co-phased symbol pairs accordingto a pattern of the cover code, wherein a range of the combining spansacross the one or more subframes; and normalizing a cost function by apower of the PSS sequence to reduce sensitivity to signal to noise ratio(SNR) levels.
 10. The method of claim 9, wherein the combining of theco-phased symbol pairs comprises: performing autocorrelation betweendelayed samples of the PSS, wherein a lag size of the delayed samples isa configurable parameter based on PSS time domain resource allocation.11. The method of claim 9, wherein a pattern of the cover code isselected to shape the ML estimation of the timing offset into a desiredform.
 12. The method of claim 1, wherein the binary sequence is acomputer generated binary sequence.
 13. The method of claim 1, whereinPSS and SSS are detected in non-overlapping resources.
 14. The method ofclaim 13, wherein PSS and SSS are detected in separate subframes. 15.The method of claim 1, wherein performing the fine timingsynchronization comprises performing cross correlation in time domain onthe SSS.
 16. The method of claim 1, wherein the first number of symbolsare mapped to resource elements that have a same subcarrier spacing asresource elements used to communicate with a second type of UE thatcommunicates on the wider system bandwidth.
 17. The method of claim 16,wherein the PSS and SSS are transmitted using resource elements not usedfor communicating with the second type of UE.
 18. The method of claim 1,wherein: the first number of symbols are within a first subframe; andthe second number of symbols are within a second subframe.
 19. Themethod of claim 1, wherein the first code sequence is generated using atleast one of a generated sequence (CGS), a modified Zadoff-Chu sequencewith a reduced alphabet, a Walsh code, a Barker code, or a truncatedZadoff-Chu sequence.
 20. The method of claim 1, wherein the second codesequence comprises a Zadoff-Chu sequence, a computer generated polyphasesequence, or a truncated Zadoff-Chu sequence.
 21. The method of claim 1,further comprising determining, from the SSS, a cell identification(cell ID) and additional system information.
 22. The method of claim 21,wherein the additional system information comprises at least a subframeID.
 23. An apparatus for wireless communications, comprising: at leastone processor configured to: detect, within one or more narrowbandregions, a primary synchronization signal (PSS) generated utilizing afirst code sequence and a cover code applied to the first code sequenceover a first number of symbols within one or more subframes, perform atime offset estimation and a frequency offset estimation based on thePSS, wherein the time offset is uncorrelated with the frequency offset,and perform an initial time synchronization and a frequencysynchronization based on the PSS.